The lone halacha in Shulchan Aruch O.C. 603:
Even someone who is not careful about eating [pas akum], during the 10 days of repentance he must be careful.
Since the S.A. didn't mention stringencies of a higher caliber, I had always applied this specifically to cases like pas akum where being strict was preferable even according to the more lenient approach. I assumed that being strict in permitted vs forbidden cases would put your past actions in a bad light.
I saw recently that the Aruch haShulchan takes a similar approach, but his logic is that acting strict in permitted vs forbidden cases is like accepting the strict approach for the future.
I have heard many prominent Rabbis tell their congregants/students to be strict during these days. As an example, even if you don't normally drink chalav yisrael...
My question is: Is there a source that applies this law to permissible/forbidden cases? If the halacha is applied broadly, why did the S.A (or the Tur) use pas akum instead of more serious issues?